Vagal nerve stimulation (VNS), therapeutic electrical stimulation of a patient's cervical vagus nerve, is used for treatment of multiple health conditions, including clinical treatment of drug-refractory epilepsy and depression. VNS has also been proposed for therapeutic treatment of heart conditions such as Congestive Heart Failure (CHF), a chronic medical condition in which the heart is unable to pump sufficient blood to meet the body's needs. For instance, VNS has been demonstrated in canine studies as efficacious in simulated treatment of atrial fibrillation (AF) and heart failure, such as described in Zhang et al., “Therapeutic Effects of Selective Atrioventricular Node Vagal Stimulation in Atrial Fibrillation and Heart Failure,” J. Cardiovasc. Electrophysiol., Vol. 24, pp. 86-91 (January 2013), the disclosure of which is incorporated by reference.
VNS therapy commonly requires an implantation of a neurostimulator, a surgery requiring several weeks of recovery before a patient can start receiving VNS therapy. Even after the recovery, the neurostimulator does not immediately start delivering a full therapeutic dose of VNS to avoid causing significant patient discomfort and other undesirable side effects. Instead, to allow the patient to adjust to the VNS therapy, the intensity is gradually up titrated over a period of time under a control of a physician. Medically, such a titration can be completed in a period not exceeding two months. The titration can take significantly longer in practice because the increase in intensity must generally be performed by a physician or another healthcare professional. Thus, for every step in the titration to take place, the patient has to visit the physician's office. These visits generally occur once every one to three months due to scheduling conflicts, and can extend the titration process to as much as a year, during which the patient in need of VNS does not receive the VNS at the full therapeutic intensity.
The medically unnecessary delays in delivering therapeutic levels of VNS can further accumulate if the therapy is disrupted following the initial titration, such as if the implanted VNS neurostimulator runs out of power. Power for a conventional VNS neurostimulator is typically provided by an onboard battery. Delivering therapeutic VNS through an implantable neurostimulator presents battery longevity concerns similar to other types of implantable pulse generators, although a VNS neurostimulator is therapeutic and non-life sustaining Still, as the duty cycle of a VNS neurostimulator can at times be near constant, battery depletion can occur at a faster rate than with cardiac pacemakers, implantable cardioverter defibrillators (ICDs) and similar implantable devices that are triggered relatively infrequently under normal conditions. The in-service lifetime of a conventional VNS neurostimulator typically varies from three to seven years, depending upon programming, particularly duty cycle and stimulation intensity. As well, an increase in lead impedance can further cause premature battery depletion. Battery depletion necessitates eventual neurostimulator explantation and replacement, with attendant surgical risks of injury and infection. The disruption in therapy caused by the surgery requires a new cycle of titration of therapeutic intensity, which as described above, can take up to a year. Thus, a patient using a conventional VNS stimulator can be subjected to repeated, prolonged, and medically unjustified delays in receiving VNS stimulation that can be harmful to the patient's health.
In addition to the challenges associated with conventional VNS stimulators described above, conventional VNS stimulators do not achieve optimum effectiveness in treating the underlying causes of CHF. CHF, as well as other forms of chronic cardiac dysfunction, are generally attributed to an autonomic imbalance of the sympathetic and parasympathetic nervous systems that, if left untreated, can lead to cardiac arrhythmogenesis, progressively worsening cardiac function and eventual patient death. Furthermore, CHF is pathologically characterized by an elevated neuroexitatory state and is accompanied by physiological indications of impaired arterial and cardiopulmonary baroreflex function with reduced vagal activity.
Experimental VNS for cardiac therapy has focused on the efferent nerves of the parasympathetic nervous system, such as described in Sabbah et al., “Vagus Nerve Stimulation in Experimental Heart Failure,” Heart Fail. Rev., 16:171-178 (2011), the disclosure of which is incorporated by reference. Sabbah discusses canine studies using a VNS system manufactured by BioControl Medical Ltd., Yehud, Israel, that includes an electrical pulse generator, right ventricular endocardial sensing lead, and right vagus nerve cuff stimulation lead. The sensing lead enables closed loop synchronization to the cardiac cycle; stimulation is delivered only when heart rate increases above a preset threshold. An asymmetric tri-polar nerve cuff electrode provides cathodic induction of action potentials while simultaneously applying asymmetric anodal blocks that lead to preferential activation of vagal efferent fibers. Stimulation is provided at an intensity and frequency intended to measurably reduce basal heart rate by ten percent by preferential stimulation of efferent vagus nerve fibers leading to the heart while blocking afferent neural impulses to the brain. The degree of therapeutic effect on parasympathetic activation occurs through incidental recruitment of afferent parasympathetic nerve fibers in the vagus, as well as through recruitment of efferent fibers.
Other uses of electrical nerve stimulation for therapeutic treatment of cardiac and physiological conditions are described. For instance, U.S. Pat. No. 8,219,188, issued Jul. 10, 2012 to Craig discloses synchronization of vagus nerve stimulation with a physiological cycle, such as the cardiac or respiratory cycle, of a patient. Electrical stimulation is applied to the vagus nerve at a selected point in the physiological cycle correlated with increased afferent conduction, such as a point from about 10 msec to about 800 msec after an R-wave of the patient's ECG, optionally during inspiration by the patient; to increase heart rate variability, such as a point from about 10 msec to about 800 msec after an R-wave of the patient's ECG, optionally during expiration by the patient; not correlated with increased efferent conduction on the vagus nerve; to generate efferent electrical activity on the vagus nerve; or upon the detection of a symptom of a medical condition. In a further embodiment, conventional VNS is applied to the vagus nerve along with microburst electrical signals, which is a portion of a therapeutic electrical signal having a limited plurality of pulses, separated from one another by interpulse intervals, and a limited burst duration, separated from one another by interburst periods. Stimulation may be applied to generate efferent electrical activity on the nerve in a direction away from the central nervous system; through a “blocking” type of electrical signal, such that both afferent and efferent electrical activity on the nerve is prevented from traveling further; or wherein afferent fibers are stimulated while efferent fibers are not stimulated or are blocked, and vice versa. By applying a series of microbursts to the vagus nerve, enhanced vagal evoked potentials (eVEP) are produced in therapeutically significant areas of the brain, in contrast to conventional VNS alone, which fails to produce eVEP.
U.S. Pat. No. 6,600,954, issued Jul. 29, 2003 to Cohen et al. discloses a method and apparatus for selective control of nerve fiber activations for reducing pain sensations in the legs and arms. An electrode device is applied to a nerve bundle capable of generating, upon activation, unidirectional action potentials that propagate through both small diameter and large diameter sensory fibers in the nerve bundle, and away from the central nervous system.
U.S. Pat. No. 6,684,105, issued Jan. 27, 2004 to Cohen et al. discloses an apparatus for treatment of disorders by unidirectional nerve stimulation. An apparatus for treating a specific condition includes a set of one or more electrode devices that are applied to selected sites of the central or peripheral nervous system of the patient. For some applications, a signal is applied to a nerve, such as the vagus nerve, to stimulate efferent fibers and treat motility disorders, or to a portion of the vagus nerve innervating the stomach to produce a sensation of satiety or hunger. For other applications, a signal is applied to the vagus nerve to modulate electrical activity in the brain and rouse a comatose patient, or to treat epilepsy and involuntary movement disorders.
U.S. Pat. No. 7,123,961, issued Oct. 17, 2006 to Kroll et al. discloses stimulation of autonomic nerves. An autonomic nerve is stimulated to affect cardiac function using a stimulation device in electrical communication with the heart by way of three leads suitable for delivering multi-chamber stimulation and shock therapy. For arrhythmia detection, the device utilizes atrial and ventricular sensing circuits to sense cardiac signals to determine whether a rhythm is physiologic or pathologic. The timing intervals between sensed events are classified by comparing them to a predefined rate zone limit and other characteristics to determine the type of remedial therapy needed, which includes bradycardia pacing, anti-tachycardia pacing, cardioversion shocks (synchronized with an R-wave), or defibrillation shocks (delivered asynchronously).
U.S. Pat. No. 7,225,017, issued May 29, 2007 to Shelchuk discloses terminating ventricular tachycardia in connection with any stimulation device that is configured or configurable to stimulate nerves, or stimulate and shock a patient's heart. Parasympathetic stimulation is used to augment anti-tachycardia pacing, cardioversion, or defibrillation therapy. To sense atrial or ventricular cardiac signals and provide chamber pacing therapy, particularly on the left side of the heart, the stimulation device is coupled to a lead designed for placement in the coronary sinus or its tributary veins. Cardioversion stimulation is delivered to a parasympathetic pathway upon detecting a ventricular tachycardia. A stimulation pulse is delivered via the lead to electrodes positioned proximate to the parasympathetic pathway according to stimulation pulse parameters based at least in part on the probability of reinitiation of an arrhythmia.
U.S. Pat. No. 7,277,761, issued Oct. 2, 2007 to Shelchuk discloses vagal stimulation for improving cardiac function in heart failure patients. An autonomic nerve is stimulated to affect cardiac function by way of three leads suitable for delivering multi-chamber endocardial stimulation and shock therapy. When the stimulation device is intended to operate as an implantable cardioverter-defibrillator (ICD), the device detects the occurrence of an arrhythmia, and applies a therapy to the heart aimed at terminating the detected arrhythmia. Defibrillation shocks are generally of moderate to high energy level, delivered asynchronously, and pertaining exclusively to the treatment of fibrillation.
U.S. Pat. No. 7,295,881, issued Nov. 13, 2007 to Cohen et al. discloses nerve branch-specific action potential activation, inhibition and monitoring. Two preferably unidirectional electrode configurations flank a nerve junction from which a preselected nerve branch issues with respect to the brain. Selective nerve branch stimulation can be used in conjunction with nerve-branch specific stimulation to achieve selective stimulation of a specific range of fiber diameters, substantially restricted to a preselected nerve branch, including heart rate control, where activating only the vagal B nerve fibers in the heart, and not vagal A nerve fibers that innervate other muscles, can be desirous.
U.S. Pat. No. 7,778,703, issued Aug. 17, 2010 to Gross et al. discloses selective nerve fiber stimulation for treating heart conditions. An electrode device is coupled to a vagus nerve and a control unit applies stimulating and inhibiting currents to the vagus nerve, which are capable of respectively inducing action potentials in a therapeutic direction in first and second sets of nerve fibers in the vagus nerve and inhibiting action potentials in the therapeutic direction in the second set of nerve fibers only. The nerve fibers in the second set have larger diameters than the first set's nerve fibers. Typically, the system is configured to treat heart failure or heart arrhythmia, such as AF or tachycardia by slowing or stabilizing the heart rate, or reducing cardiac contractility.
U.S. Pat. No. 7,813,805, issued Oct. 12, 2010 to Farazi and U.S. Pat. No. 7,869,869, issued Jan. 11, 2011 to Farazi both disclose subcardiac threshold vagus nerve stimulation. A vagus nerve stimulator is configured to generate electrical pulses below a cardiac threshold, which are transmitted to a vagus nerve, so as to inhibit or reduce injury resulting from ischemia. For arrhythmia detection, a heart stimulator utilizes atrial and ventricular sensing circuits to sense cardiac signals to determine whether a rhythm is physiologic or pathologic. In low-energy cardioversion, an ICD device typically delivers a cardioversion stimulus synchronously with a QRS complex. If anti-tachycardia pacing or cardioversion fails to terminate a tachycardia, after a programmed time interval or if the tachycardia accelerates, the ICD device initiates defibrillation.
U.S. Patent App. Pub. No. 2010/0331908, filed Sep. 10, 2010 by Farazi discloses subcardiac threshold vagus nerve stimulation in which a vagal nerve stimulator generates electrical pulses below a cardiac threshold of the heart for treating an ischemia of the heart, or for reducing a defibrillation threshold of the heart. The cardiac threshold is a threshold for energy delivered to the heart above which there is a slowing of the heart rate or the conduction velocity. In operation, the vagal nerve stimulator generates the electrical pulses below the cardiac threshold, that is, subcardiac threshold electrical pulses, such that the beat rate of the heart is not affected. Although the function of the vagal nerve stimulator is to treat an ischemia, or to reduce a defibrillation threshold of the heart, in other embodiments, the vagal nerve stimulator may function to treat heart failure, reduce an inflammatory response during a medical procedure, stimulate the release of insulin for treating diabetes, suppress insulin resistance for treating diabetes, or treat an infarction of the heart.
U.S. Pat. No. 7,634,317, issued Dec. 15, 2009, to Ben-David et al. discloses techniques for applying, calibrating and controlling nerve fiber stimulation, which includes a vagal stimulation system comprising a multipolar electrode device that is applied to a portion of a vagus nerve (a left vagus nerve and/or right vagus nerve), which innervates a heart of a subject. Alternatively, the electrode device is applied to an epicardial fat pad, a pulmonary vein, a carotid artery, a carotid sinus, a coronary sinus, a vena cava vein, a right ventricle, or a jugular vein. The system is utilized for treating a heart condition such as heart failure and/or cardiac arrhythmia; the vagal stimulation system further comprises an implantable or external control unit, which typically communicates with electrode device over a set of leads. Typically, the control unit drives the electrode device to (i) apply signals to induce the propagation of efferent nerve impulses towards heart, and (ii) suppress artificially-induced afferent nerve impulses towards a brain of the subject, to minimize unintended side effects of the signal application; the efferent nerve pulses in vagus nerve are typically induced by the electrode device to regulate the heart rate of the subject.
Finally, U.S. Pat. No. 7,885,709, issued Feb. 8, 2011 to Ben-David discloses nerve stimulation for treating disorders. An electrode device stimulates the vagus nerve, so as to modify heart rate variability, or to reduce heart rate, by suppressing the adrenergic (sympathetic) system. Typically, the system is configured to treat heart failure or heart arrhythmia. Therapeutic effects of reduction in heart rate variability include the narrowing of the heart rate range, thereby eliminating very slow heart rates and very fast heart rates. For this therapeutic application, the control unit is typically configured to reduce low-frequency heart rate variability, and to adjust the level of stimulation applied based on the circadian and activity cycles of the subject. Therapeutic effects also include maximizing the mechanical efficiency of the heart by maintaining relatively constant ventricular filling times and pressures.
Notwithstanding, a need remains for an approach to therapeutically treating chronic cardiac dysfunction through a neurostimulator that allows to avoid medically unjustified delays in VNS therapy associated with titration of intensity of VNS stimulation.